Laser crystallization of an amorphous silicon film that has been deposited on a substrate, e.g., glass, represents a promising technology for the production of material films having relatively high electron mobilities. Once crystallized, this material can then be used to manufacture thin film transistors (TFT's) and in one particular application, TFT's suitable for use in relatively large liquid crystal displays (LCD's). Other applications for crystallized silicon films may include Organic LED (OLED), System on a Panel (SOP), flexible electronics and photovoltaics. In more quantitative terms, high volume production systems may be commercially available in the near future capable of quickly crystallizing a film having a thickness of about 90 nm and a width of about 700 mm or longer.
Laser crystallization may be performed using pulsed laser light that is optically shaped to a line beam, e.g., laser light that is focused in a first axis, e.g., the short-axis, and expanded in a second axis, e.g., the long-axis. Typically, the first and second axes are mutually orthogonal and both axes are substantially orthogonal to a central ray traveling toward the film. An exemplary line beam for laser crystallization may have a beam width at the film of less than about 20 microns, e.g. 3-4 microns, and a beam length of about 700 mm. With this arrangement, the film can be scanned or stepped in a direction parallel to the beam width to sequentially melt and crystallize a film having a substantial length, e.g., 900 mm or more.
In some cases, e.g. sequential lateral solidification processes, it may be desirable to ensure that the silicon film is exposed using a beam having an intensity that is relatively uniform across the short-axis and that drops off sharply at the short-axis edges (i.e. a beam having relatively steep, short-axis sidewalls). More specifically, failure to obtain a steep sidewall on the trailing short-axis edge may result in the undesirable crystal quality of new grains near the short-axis edge due to insufficient overlap between adjacent pulses. Also, in some implementations, it may be desirable to have a steep sidewall on the leading short-axis edge to reduce surface variations and provide more consistent lateral growth. One way to achieve this desired beam shape is to focus a laser beam at a short-axis stop, e.g. field stop, which is shaped as an elongated slit and aligned in the direction of the long-axis. An optic may then be used to produce an image of the short-axis stop at the film. With this arrangement, a beam having relatively steep, short-axis sidewalls may be obtained.
In some cases, it may be desirable to ensure that each portion of the silicon film is exposed to an average laser energy density that is controlled within a preselected energy density range during melting. In particular, energy density control within a preselected range is typically desired for locations along the shaped line beam, and a somewhat constant energy density is desirable as the line beam is scanned relative to the silicon film. High energy density levels may cause the film to flow resulting in undesirable “thin spots”, a non-flat surface profile and poor grain quality. This uneven distribution of film material is often termed “agglomeration” and can render the crystallized film unsuitable for certain applications. On the other hand, low energy density levels may lead to incomplete melting and result in poor grain quality. By controlling energy density, a film having substantially homogeneous properties may be achieved.
Laser beam homogenization is a common practice used in various laser applications such as lithography to produce a beam having a fairly uniform intensity across the beam. However, for applications such as the one described above which contemplate a line beam having a long-axis length of 700 mm or more, common methods of beam homogenization may be insufficient to produce a beam having suitable uniformity. More specifically, past methods have primarily used monolithic lenslet arrays (so-called fly's eye arrays) in combination with a Fourier lens to produce a somewhat uniform beam in the far field of the Fourier lens. However, the individual lenses of the lenslet array often contain defects, and thus, do not bend the incoming laser light in a desired manner. For the case when the type and location of the defects are randomly distributed throughout the array, the impact of the defects may not be very severe because they tend to average out. On the other hand, when each lens in the array contains the same type of defect in the same location, the results do not average out, and instead, the result may be an inhomogeneous beam unsuitable for use as a 700 mm long line beam. Moreover, due to manufacturing techniques that are typically employed to produce monolithic lenslet arrays, (polishing from a single piece of glass) these non-random defects are often present in the arrays rendering these prior art homogenization systems unsuitable for certain applications.
With the above in mind, applicants disclose systems and methods for shaping laser light as a homogeneous line beam for interaction with a film deposited on a substrate.